If a mobile device is located in a power pattern null of the base station (BS) transmitter antenna, the mobile device will not receive the signal being transmitted. In order to solve this problem, each call between a mobile device and the base transceiver station (BTS) is handled by two transmitters at the BTS, each of which is connected to a respective antenna. The transmitters transmit the same information, but the transmission by one transmitter is delayed by one quarter of a symbol with respect to the transmission by the other. This delay ensures that if the mobile device is located in the null of the antenna of one of the transmitters handling the call, the mobile device will not also be located in the null of the antenna of the other transmitter handling the call. Such transmit delay diversity improves signal reception in the downlink direction.
BTSs that provide downlink transmit delay diversity (TDD) normally allocate a pair of transmitters to handle each call. For example, if each BTS transmitter has eight time slots for handling eight different calls, then a pair of transmitters should have sixteen time slots for handling sixteen calls. However, in order to provide downlink TDD, each call must be handled by two transmitters. Therefore, each pair of transmitters only has eight time slots and thus can only handle eight different calls. Consequently, twice as many transmitters and antennas are needed to provide downlink TDD than are needed when downlink TDD is not provided.
A need exists for a way to provide TDD in the downlink direction that reduces the overall number of transmitters and antennas that are needed at the BTS.